1. Field of the Invention
This invention relates in general to electron guns for cathode ray tubes and particularly to an improved electron gun for high resolution color cathode ray tubes and also to beamindex color cathode ray tubes.
2. Description of the Prior Art
High resolution color cathode ray tubes require an electron gun which has an excellent beam focusing characteristic. Thus, previously there have been proposed various types of electron guns having large-aperture anodes so as to reduce the spherical aberration of the electron lens, particularly of the principal lens. There is known, for example, a large aperture electron gun in which a conductive coating is applied to the inside of the bulb neck portion of the envelope which serves as the second anode but such type tube is not reliable because the neck of the tube is not round and, thus, there will be eccentricity between the first and second anodes of this type of tube.
There has also been proposed an electron gun having a large aperture anode such as illustrated in FIG. 3. In FIG. 3, the electron gun has a cathode K which is supported by common insulating support members 3. Also, mounted in the electron gun is a control electrode G1 of generally cylindrical-shape with an opening in its end face and an accelerating electrode G2 of generally cylindrical shape with an opening in its end face. A first anode G3 is formed of a generally cylindrical-shaped portion 5 of smaller diameter than its second portion 2 which has a larger diameter. The electrodes G1 and G2 and the first anode G3 are supported from the supporting members 3 and the first anode G3 is supported from the support members 3 in the small diameter portion 5 as illustrated. A second anode G4 has a large diameter cylindrical portion 1 which extends around the portion 2 of the first anode and also extends beyond the end of the enlarged portion 2. The second anode G4 also has a smaller diameter portion 17 which is concentric with the smaller diameter portion 5 of the first anode G3 and the second anode G4 is supported from the insulating support members 3 which have supports that extend to the small diameter portion 17 of the second anode.
The electron gun illustrated in FIG. 3 has a first anode G3 in which the cross-sectional dimension remains close to a perfect circle. However, since the first anode G3 is supported at its small diameter portion 5 from the support members 3 and also since the point of support of the small diameter 5 is a relatively great distance from the principal lens forming portion which is between the portions 2 and 1 and also since the first anode G3 is rather long and the large diameter portion 2 which forms a part of the principal lens is rather heavy, mechanical instability exists and it is very difficult to obtain and maintain symmetry between the large portion 2 of the first anode G3 and the large portion 1 of the second anode G4.